RF Circuit with Improved Antenna Matching

ABSTRACT

In one embodiment, RF front-end circuit includes a tunable matching network having an input coupled to an RF interface port, a directional coupler with a first connection coupled to an RF input of a mixer, a second connection coupled to an RF signal generation port, and a third connection coupled to an output of the tunable matching network. The directional coupler is configured to direct a signal from the RF signal generation port to the tunable matching network and to direct a signal from the tunable matching network port to the RF port of the mixer. The RF front-end circuit also has a tunable matching network control unit coupled to the tunable matching network. The control unit is configured to optimize an impedance match between the RF interface port and the output of the tunable matching network.

This application is a continuation of U.S. patent application Ser. No.12/248,573, entitled “RF Circuit with Improved Antenna Matching,” filedon Oct. 9, 2008 which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The invention relates generally to radio frequency circuit devices andmethods and more particularly to RF circuits with improved matching toexternal loads.

BACKGROUND

Radar systems used for distance measurement in vehicles essentiallycomprise two separate radar subsystems operating in different frequencybands. For distance measurements in a short range (short range radar),radar systems currently used are typically those which operate in afrequency band around a mid-frequency of 24 GHz. Short range typicallymeans distances from 0 to about 20 meters from the vehicle. Thefrequency band from 76 GHz to 77 GHz is currently used for distancemeasurements in the long range, which is for measurements in the rangefrom about 20 meters to around 200 meters (long-range radar). Thesedifferent frequencies are an impediment to the creation of a singleconcept for a radar system which can carry out measurements in aplurality of range zones, and in principle result in the need for twoseparate radar systems.

The frequency band from 77 GHz to 81 GHz is likewise suitable for shortrange radar applications, and has also been made available by theauthorities for this purpose, so that a frequency range from 76 GHz to81 GHz is now available for automobile radar applications in the shortrange and in the long range. A single multirange radar system whichcarries out distance measurements in the short and in the long rangeusing a single radio-frequency transmission/reception module (RFfront-end) has, however, not yet been feasible for various reasons. Onereason is that circuits manufactured using III/V semiconductortechnologies (for example gallium-arsenide technologies) are used now toconstruct known radar systems. Gallium-arsenide (GaAs) technologies arehighly suitable for the integration of radio-frequency components, butit is generally not possible to achieve a degree of integration which isas high, for example, as that which would be possible with siliconintegration because of technological restrictions. Furthermore, only aportion of the required electronics is manufactured using GaAstechnology, so that a large number of different components are requiredto construct the overall system.

RF oscillators manufactured using SiGe-technology tunable throughout theentire frequency range from 76 GHz to 81 GHz have become possiblebecause of the latest manufacturing technologies. These technologiesallow for the production of radar systems, which are substantially morecompact and more cost effective, compared to known radar systems. Besidea compact architecture, a large “field of view” of the radar sensor isdesired when designing RF front-ends of radar systems, where thetransmitted signal power increases with an increasing field of view.

Monostatic radar systems, which have common antennas for transmittingand receiving signals, are used because of their compact architecture.The RF front end of a monostatic radar system typically has adirectional coupler (e.g., a rat race coupler) for separating thesignals to be transmitted from the received signals. A received signalis down-converted to a baseband or to an intermediate frequency band(IF-band) by a mixer, which is connected to the directional coupler. Thebaseband signal or the intermediate frequency signal (IF-signal) beingprovided at the output of the mixer may be digitized for further digitalsignal processing.

A real directional coupler, which may be realized using microstriplines, does not achieve ideal properties with respect to through-lossand isolation, which ideally is zero or infinity, depending on the pairof ports of the directional coupler. The oscillator signal which issupplied to an input-port of the directional coupler for beingtransmitted is not only coupled to the port which is connected to theantenna, but a small part of the oscillator signal (which means afraction of the power of the oscillator signal) is also coupled to theport which is connected to a signal input of the mixer. This part of theoscillator signal is superimposed with the signal received by theantenna at the mixer input. This superimposition results in a DC signaloffset at the output of the mixer which is superimposed with thebaseband-signal or the IF-signal respectively. Especially when usingactive mixers this DC signal-offset can be very disturbing. The DCsignal offset increases with an increasing transmitting power.Consequently, the DC signal-offset is a parameter limiting the power ofthe signal to be transmitted and therefore limiting the field of view ofthe radar sensor.

Even in the case of an ideal directional coupler, DC signal-offset isstill a potential problem in monostatic radar systems because ofimpedance mismatch at the antenna port. In the presence of antennamismatch, any oscillator signal intended to be transmitted will bereflected at the antenna port. The reflected oscillator signal will thenbe coupled to the signal input of the mixer and cause a DCsignal-offset.

Because quarter wavelengths in the 76 GHz to 81 GHz band are less than500 μm on silicon substrates, however, obtaining a precise match betweenan antenna and an RF integrated circuit is very difficult to achieve ina high volume manufacturing environment. What are needed are systems andmethods for antenna matching in high frequency monostatic RF systems.

SUMMARY OF THE INVENTION

In one embodiment an RF front-end circuit includes a tunable matchingnetwork comprising an input coupled to an RF interface port, and adirectional coupler comprising a first connection coupled to an RF inputof a mixer, a second connection coupled to an RF signal generation port,and a third connection coupled to an output of the tunable matchingnetwork. The directional coupler is configured to direct a signal fromthe RF signal generation port to the tunable matching network and directa signal from the tunable matching network port to the RF port of themixer. The RF front-end circuit also comprises a tunable matchingnetwork control unit coupled to the tunable matching network. Thecontrol unit is configured to optimize an impedance match between the RFinterface port and the output of the tunable matching network.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a radar system according to an embodiment of thepresent invention;

FIG. 2 illustrates an RF front-end circuit using a rat-race coupleraccording to an embodiment of the present invention;

FIGS. 3 a-3 b illustrate an embodiment coupled line coupler and anembodiment tunable matching network;

FIG. 4 illustrates an RF front-end circuit using a plurality of rat-racecouplers according to an embodiment of the present invention;

FIG. 5 illustrates an RF front-end circuit using a plurality of rat-racecouplers and mixer DC feedback according to an embodiment of the presentinvention; and

FIG. 6 illustrates an RF front-end circuit using a plurality of Langecouplers and mixer DC feedback according to an embodiment of the presentinvention.

Corresponding numerals and symbols in different figures generally referto corresponding parts unless otherwise indicated. The figures are drawnto illustrate clearly the relevant aspects of embodiments of the presentinvention and are not necessarily drawn to scale. To more clearlyillustrate certain embodiments, a letter indicating variations of thesame structure, material, or process step may follow a figure number.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely a semiconductor RF front-endcircuit used in a radar system. The invention may also be applied,however, to other systems such as monostatic RF systems, RFcommunication systems or RF systems where transmission and receptionoccur at the same frequency or at frequencies very close together.

FIG. 1 illustrates a radar system 2 according to an embodiment of thepresent invention. Radar system 2 includes an RF integrated circuit 10containing an integrated radar system connected to external antenna 12.Integrated radar system 2 includes oscillator 22 for generating signalOSC to be transmitted and signal LO for mixer 24. Directional coupler 18couples signals traveling from oscillator 22 to the antenna 12, andsignals traveling from the antenna 12 to mixer 24. In variousembodiments of the present invention, radar system controller 26controls oscillator 22 and automatic matching network 20, as well asprocessing signal IF from the mixer 24. In some embodiments, RFintegrated circuit 10 is fabricated on a silicon wafer using a silicongermanium (SiGe) bipolar process; however, in alternative embodiments ofthe present invention, other types of wafers, such as gallium arsenide(GaAs) can be used. In further alternative embodiments, antenna 12 maybe implemented as an on-chip antenna.

Radar system 2 generates a radio signal with transmitted power P_(i)from antenna 12. When the radio signal reaches object 28 (such as anautomobile), a portion of the transmitted power is reflected. Thereflected signal power P_(r) is received by antenna 12, down convertedto signal IF, and processed by radar system controller 26. Processeddata from radar system controller can then be used to determine thespeed and distance of object 28 to antenna 12. In automotiveapplications, this speed and distance data may be used, for example, tomeasure the relative speed of a car in front of a driver for the purposeof controlling an automatic cruise control, or to measure the presenceof an obstacle in a blind spot while changing lanes or backing up.

Because of the high frequencies used in embodiments of the presentinvention, RF integrated circuit 10 can be bonded directly to a circuitboard (not shown) in order to reduce connection parasitics. Antenna 12can be bonded directly to output pad 16 with a short length of bondwire14.

It can be seen that even if directional coupler 18 is an ideal coupler,(i.e., there is no coupling from port A to port C) DC offset at theoutput of mixer 24 is possible if the reflection coefficient Γ_(r) atthe antenna interface (bondpad 16) is non-zero. For example, signal OSCis coupled from port A to port B of directional coupler 18. A portion ofthe coupled OSC signal is reflected at the antenna interface (bondpad16) and travels back into the circuit. The directional coupler passesthe reflected OSC signal to mixer 24. Because the mixer is beingoperated with signal LO, which is the same frequency as signal OSC, anyreflected signal generated by oscillator 22 is mixed down to DC.Excessive DC offset at the output of mixer 24 is disadvantageous becauseit desensitizes the RF front-end by reducing the available amount ofdynamic range. In extreme cases, reflected power can even saturate themixer 24. The DC offset reduces the maximum input signal power thesystem can handle, as well as possibly causing the RF circuitry tooperate in regions having suboptimal noise and distortion performance.

In various embodiments of the present invention, reflection coefficientΓ_(r) at the antenna interface (bondpad 16) giving rise to increase DCoffset is reduced by dynamically matching antenna 12 with automaticmatching network 20.

Turning to FIG. 2, an RF front-end 100 according to an embodiment of thepresent invention is shown. The RF front end circuit has an antenna 102,a tunable matching network 104, directional coupler 105, oscillator 120,power divider 118, amplifier 116, mixer 122, coupler 110, powerdetectors (represented by diodes 106 and 108), and reactance correctioncontrol unit 112. Directional coupler 105 is shown as a rat-racecoupler; however, other coupler architectures can be used in alternativeembodiments of the present invention. The signal path and components ofRF front-end 100 can comprise differential signal paths.

Dimensions of rat-race coupler 105 are chosen for the frequency band ofinterest. Coupler 105 has a total circumference of 3λ/2 in oneembodiment. For the frequency band of between 76 GHz and 81 GHz,wavelength λ is between about 2000 μm and about 1875 μm on a siliconsubstrate. For the purpose of circuit implementation, a value of about1960 μm is chosen for λ. In alternative embodiments of the presentinvention, other values of λ can be used. Segment 113 can be 3λ/4, andsegments 107, 109 and 111 can be λ/4 each. The impedance of these lines√{square root over (2)}*Z₀, where Z₀ is a system impedance, which isabout 50Ω in various embodiments. While the illustration shows acircular coupler, the coupler 105 can, in fact be implemented as afolded structure. In various embodiments of the present invention,coupler 105 is implemented as a 70Ω line in a top level of metal over aground line in a lower level of metal. Resistor 124 is 50Ω in oneembodiment. Because of the high frequency and high performancesensitivity to matching, this coupler 105 is implemented differentiallyin one embodiment. In alternative embodiments of the present invention,other frequency ranges, coupler dimensions, coupler architectures,impedance values, and physical dimensions can be used.

The operation of rat-race coupler 105 is achieved by using phase shiftsaround coupler 105 to create constructive or destructive interference.For example, signal S3 traveling into coupler 103 at port D is splitinto two components, one of which travels clockwise in the direction ofsegment 113 and counter clockwise in the direction of segment 111. Thetwo components constructively interfere at port A (which is coupled toantenna 102) because each component travels a distance of 3λ/4. At portB (which is coupled to mixer 122), however, the components destructivelyinterfere because the clockwise component has traveled a distance of λand the counter-clockwise component has traveled a distance of λ/2.Because of the difference of one-half wavelength, each component is 180°out of phase with each other, and the superposition of the two signalsideally renders no signal at port B. Using a similar analysis, thesignal traveling into port D constructively interferes at port C.Therefore, resistor 124 absorbs half the power from amplifier 116.

Signals traveling into port A, however, constructively interfere at portB because the clockwise and counter-clockwise paths are in-phase witheach other. Because of this constructive interference, any signalreflected at antenna 102 will be reflected to port A and out port B intomixer 122.

In various embodiments of the present invention, tunable matchingnetwork 104 is coupled to the antenna port. A measurement of theimpedance mismatch or reflection at the tunable matching network 104 ismade along a length of transmission line between matching network 104and port A of coupler 105. In various embodiments of the presentinvention, coupler 110 is used to incident and reflected power at theantenna interface to power detectors represented by detection diodes 106and 108. Power detectors represented by detection diodes 106 and 108 canbe implemented by known techniques. Reactance control unit 112 processesthe output of detectors 106 and 108 and outputs control signal MN_CONTthat controls tunable matching network 104.

In various embodiments of the present invention, coupler 110 isimplemented using a single section coupled line coupler, as shown inFIG. 3 a. Single section line coupler 110 has two adjacent lines 190 and192 in close proximity for a distance 119, which is preferably λ/4. Inalternative embodiments of the present invention, lengths of less thanλ/4 can be used to save space as long as the corresponding change inbehavior is taken into account. In various embodiments of the presentinvention where the frequency band of 77 GHz to 81 GHz is used, length119 is about 490 um. Port Al is coupled to coupler 105 (FIG. 2), port B1is coupled to tunable matching network 104 (FIG. 2), and ports C1 and D1are coupled to detectors 106 and 108 (FIG. 2) respectively. A waveincident to antenna 102 travels from port A1 to port B1, and a portionof the incident wave power is coupled to port C1. A wave reflected fromantenna 102 travels from port B1 to port A1 and a portion of thereflected power is coupled to port D1.

Turning back to FIG. 2, the reactance correction control unit 112 inputsmeasures of the incident power from detector 106 and the reflected powerfrom detector 108 and determines a figure of merit for the amount ofpower reflected by antenna 102. In various embodiments of the presentinvention, reactance correction control unit 112 derives a figure ofmerit related to the reflection coefficient; however, in alternativeembodiments of the present invention, other figures of merit can be usedsuch as a voltage standing wave ratio (VSWR), or a reflected powermeasurement. Reactance correction control unit 112 can be implementedusing a network of analog summers and amplifiers, or a digitalimplementation can be used. In various embodiments of the presentinvention, detectors 106 and 108 are simple diode power detectors,however, in alternative embodiments of the present invention, detectors106 and 108 could each output a signal proportional to the log of thedetected powers and a simple subtraction could yield a valueproportional to a dB return loss.

Once the reactance correction control unit 112 derives a figure ofmerit, a control signal is output to the tunable matching network inorder to optimize the figure of merit. In various embodiments of thepresent invention, this optimization is performed by a feedback loop,where the control signal MN_CONT functions as an error signal and thefigure of merit functions as the control variable. In alternativeembodiments, the control signal can be increased or decreased until thefigure of merit reaches a first preset threshold. In some embodiments,once the threshold is reached, further updates to the control signal canbe disabled until the figure of merit degrades beyond a secondthreshold, in which case the control signal is modified accordingly. Thefirst and second threshold can be selected to add hysteresis to thereactance control system in order to reduce disturbances duringoperation.

Turning to FIG. 3 b, a schematic of an embodiment of tunable matchingnetwork 104 is shown. In various embodiments, a pi LC network having aseries inductor L, programmable shunt capacitors C₁ and C₂, and ports130 and 132 are used. In various embodiments of the present invention,the nominal values of these are selected to compensate bond padcapacitance. Programmable capacitors C₁ and C₂ can be voltage-controlledcapacitors implemented by a varactor diode, a junction diode, or MOSFETcapacitor, controlled by an analog MN_CONT signal. In variousembodiments, shunt capacitors C₁ and C₂ may be controllableindependently to achieve better matching. In alternative embodiments ofthe present invention, a switched array of individual capacitorscontrolled by a digital representation of signal MN_CONT can be used. Infurther alternative embodiments, other suitable programmable matchingnetwork structures and programmable element types can also be used.

Turning to FIG. 4, another embodiment of the present invention isillustrated. The embodiment of FIG. 4 is similar to the embodiment ofFIG. 2, except that two directional couplers 142 and 140 and a powercombiner 150 are used instead of a single coupler, and dual-outputamplifier 152 is used instead of a single output amplifier. In anembodiment of the present invention, the oscillator signal is amplifiedby amplifier 152 and output as signals S3P and S3B that are 180° out ofphase from each other.

In various embodiments, amplifier 152 is implemented as a differentialamplifier, however in alternative embodiments a multiplicity of singleended amplifiers can be used depending on the application. As seen inFIG. 4, the amplifier 152 is fed with signals S1 and S2. If theoscillator has a differential architecture, signals S1 and S2 are out ofphase 180° with each other. If the oscillator is single ended, however,a ring hybrid can be used to create the two out of phase signals S1 andS2. In further embodiments, other techniques can be used to create S1and S2. Any systematic residual oscillator components remaining at portsB of directional couplers 140 and 142 are summed together and input tomixer 122. Because the output ports B of each coupler 140 and 142 are180° out of phase with each other, these components will cancel eachother out, thereby reducing the oscillator signal seen at node RF1.

Power combiner 150 can be implemented with a Wilkinson combiner;however, other architectures can be used. The signal path that includespower splitter 118, amplifier 152, directional couplers 140 and 142,amplifier, and power combiner 150 should be well matched in the layoutin order to ensure good phase matching of signals S3P and S3B and,therefore, adequate cancelation of the residual oscillator signals atthe input to mixer 122.

Turning to FIG. 5, a further embodiment of the present invention isillustrated. The embodiment of FIG. 5 is similar to the embodiment ofFIG. 4, with the addition of additional control inputs and outputs toreactance control unit 112. Signal DC representing the DC offset ofmixer 122 is input to the reactance correction control unit 112. In anembodiment of the present invention, DC offset is used as a figure ofmerit in addition to VSWR at the antenna interface in order to determineand control the effectiveness of the tunable matching network 104.Furthermore, the directivity of the directional coupler made fromcouplers 140 and 142 can be improved. Programmable impedance 162 inseries with resistor 164 terminates port A of coupler 142. By adjustingthe termination of this port, the balance of the directional coupler canbe adjusted. In various embodiments of the present invention, both theVSWR and the DC offset of mixer 122 can be simultaneously optimized byadjusting tunable matching network 104 and programmable impedance 162 tooptimize a combined figure of merit. For example, the combined figure ofmerit may be calculated by summing the mean-square values of the VSWRand the DC offset. In various embodiments of the present invention, anLMS algorithm can be used to optimize the system's performance.Alternatively, other optimization schemes and algorithms can be used.

FIG. 6 illustrates an embodiment of the present invention similar toFIG. 5, except that Lange couplers 182 and 184 are used instead ofrat-race couplers. Lange couplers can be designed to have a widerbandwidth than a corresponding rat-race coupler. Furthermore, Langecouplers may enable more design flexibility because not all ports of theLange coupler are DC coupled. In alternative embodiments of the presentinvention, other coupler architectures can be used in place of Langecouplers or rat-race couplers.

Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be obvious to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted.

1. An RF front-end circuit comprising: a tunable matching networkcomprising an input coupled to an RF interface port; a directionalcoupler comprising a first connection coupled to an RF input of a mixer,a second connection coupled to an RF signal generation port, and a thirdconnection coupled to an output of the tunable matching network, whereinthe directional coupler is configured to direct a signal from the RFsignal generation port to the tunable matching network and direct asignal from the tunable matching network to the RF input of the mixer;and a tunable matching network control circuit coupled to the tunablematching network, the control circuit configured to adjust an impedancematch between the RF interface port and the output of the tunablematching network based on signal measurements made during operation ofthe RF front-end.
 2. The RF front-end circuit of claim 1, wherein: thesignal measurements comprise reflection measurements; the RF front-endfurther comprises a reflection measurement circuit coupled to the outputof the tunable matching network; and the tunable matching networkcontrol circuit is further configured to reduce a reflection at theoutput of the tunable matching network based on the reflectionmeasurements.
 3. The RF front-end circuit of claim 2, wherein thereflection measurement circuit is configured to measure reflected power.4. The RF front-end circuit of claim 2, wherein the reflectionmeasurement circuit comprises a plurality of signal level detectorscoupled to a length of transmission line at the output of the tunablematching network.
 5. The RF front-end circuit of claim 2, wherein thetunable matching network control circuit is further configured to adjustthe tunable matching network to minimize a DC offset at an output of themixer.
 6. The RF front-end circuit of claim 1, wherein the signalmeasurements comprise voltage standing wave ratio (VSWR) measurements;the RF front-end further comprises a VSWR measurement circuit coupled tothe output of the tunable matching network; and the tunable matchingnetwork control circuit is further configured to adjust the tunablematching network based on the VSWR measurements.
 7. The RF front-endcircuit of claim 1, wherein the tunable matching network control circuitis configured to adjust the tunable matching network to minimize a DCoffset at an output of the mixer.
 8. The RF front-end circuit of claim1, wherein the tunable matching network, directional coupler and tunablematching network are disposed on an integrated circuit.
 9. A integratedcircuit comprising: a system input; a tunable matching networkcomprising an input coupled to the system input; a detection circuitcoupled to an output of the tunable matching network; and a reactancecontrol circuit coupled to a measurement output of the detection circuitand a control input of the tunable matching network, wherein thereactance control circuit is configured to reduce a reflection at theoutput of the tunable matching network based on measurements made by thedetection circuit during operation of the integrated circuit; and adirectional coupler coupled between a signal generator and the output ofthe matching network.
 10. The integrated circuit of claim 9, furthercomprising a directional coupler coupled between a signal generator andthe output of the matching network, the a directional coupler configuredto directionally couple a transmission signal from a signal generator tothe output of the matching network, and directionally couple an antennainput signal from the output of the matching network to an input of amixer.
 11. The integrated circuit of claim 10, wherein the directionalcoupler is further configured to directionally attenuate thetransmission signal from the signal generator to the input of the mixer.12. The integrated circuit of claim 10, further comprising: a powersplitter comprising an input coupled to the signal generator, and afirst output coupled to an LO input of the mixer; and an RF amplifiercomprising a first input coupled to a second output of the powersplitter, a second input coupled to a third output of the powersplitter, and an output coupled to the directional coupler.
 13. Theintegrated circuit of claim 12, further comprising: a further powersplitter coupled between the directional coupler and the input of themixer, wherein a first input of the power splitter is coupled to thedirectional coupler and an output of the power splitter is coupled tothe input of the mixer; and a further directional coupler configured todirectionally couple an inverted output of the RF amplifier to a firstinput of a further power splitter.
 14. The integrated circuit of claim13, wherein the directional coupler and the further directional couplerare adjusted to null out a signal traveling from the signal generator tothe input of the mixer.
 15. The integrated circuit of claim 9, wherein:the system input comprises an antenna input; and the integrated circuitcomprises a radar front-end circuit comprising the tunable matchingnetwork, detection circuit, and reactance control circuit.
 16. Theintegrated circuit of claim 9, wherein the detection circuit isconfigured to detect a reflection or an impedance mismatch.
 17. A methodof operating a RF system, the method comprising: measuring a reflectionon a transmission line coupled to an input port during operation of thesystem; adjusting a matching network coupled to the input port duringoperation of the system, wherein the reflection is reduced based on themeasuring; directionally coupling the input port to a mixer; generatinga reference signal; coupling the reference signal to an LO port of themixer; modulating the reference signal forming a modulated referencesignal; and directionally coupling the modulated reference signal to theinput port.
 18. The method of claim 17, wherein: the reference signalcomprises a continuous wave signal, and modulating the reference signalcomprises pulsing the reference signal.
 19. The method of claim 17,wherein measuring the reflection comprises coupling a plurality of powerdetectors along a length of the transmission line.
 20. The method ofclaim 17, wherein: the input port comprises an antenna port; and the RFsystem is a radar system.